Method for the electrochemical deposition of carbon nanotubes

ABSTRACT

This invention relates to the electrochemical deposition of carbon nanotubes (“CNTs”) on a substrate using an electrochemical cell. A dispersion of a complex of CNTs and an anionic polymer is neutralized and thereby caused to deposit on the anode plate of the cell.

This application claims the benefit of U.S. Provisional Application No.60/903,260, filed 24 Feb. 2007, which is by this reference incorporatedin its entirety as a part hereof for all purposes.

TECHNICAL FIELD

This invention relates to the electrochemical deposition of carbonnanotubes (“CNTs”) on a substrate.

BACKGROUND

U.S. Pat. No. 6,902,658 describes an electrophoretic deposition methodin which a separate step of depositing a binder material onto asubstrate is performed prior to deposition thereon of CNTs. A need thusremains for a method in which CNTs and one or more accompanyingmaterials may be deposited onto a substrate simultaneously.

SUMMARY

In one embodiment, this invention provides a method for the depositionof carbon nanotubes by:

(a) providing an electrochemical cell that comprises a cathode, an anodeplate, a first electrically conducting pathway connecting the cathode toan electrical power supply, and a second electrically conducting pathwayconnecting the electrical power supply to the anode plate;

(b) providing as an aqueous electrolyte disposed between the cathode andthe anode a dispersion of a complex formed from carbon nanotubes and afirst anionic polymer; and

(c) applying a voltage to the electrochemical cell to deposit thecomplex on the anode.

In another embodiment, this invention provides a film that includes asubstrate and, disposed on the substrate, (a) coagulant residue, and (b)a complex formed from carbon nanotubes and a first anionic polymer.

In a further embodiment, this invention provides a cathode assembly fora field emission device comprising a film as described above.

In yet another embodiment, this invention provides a field emissiondevice comprising a cathode assembly as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of the mechanism of deposit inone embodiment of the methods of this invention.

FIG. 2 shows deposited material on a film as prepared in Example 1.

FIG. 3 shows the configuration of an electrochemical cell as used in theexamples.

FIG. 4 shows deposited material on a film as prepared in Example 2.

FIG. 5 shows an image of phosphor illumination from a film as tested inExample 3.

FIG. 6 shows a plot of recorded anode current and anode voltage valuesas obtained in Example 4.

FIG. 7 shows an image of phosphor illumination from a film as tested inExample 4.

FIG. 8 shows an image of phosphor illumination from a film as tested inExample 5.

DETAILED DESCRIPTION

CNTs are well known to have unique and useful electrical properties, andare frequently used in the fabrication of the cathode of a fieldemission device. However, adoption of these materials is constrained bytheir high cost. Therefore, an objective of the invention is to providea process for making a uniform CNT film on a substrate such as aconducting substrate with good uniformity and low material consumption.A further objective is to pattern the CNT film thus prepared to beuseful in electronic applications. The CNT film so made may be used in acathode assembly that is installed in a field emission device.

A CNT film is made by the method of this invention by the deposition ofCNTs on a substrate by electrochemical means, and for such purpose themethod hereof involves the use of an electrochemical cell. The cellcontains a cathode, an anode plate, a first electrically conductingpathway connecting the cathode to an electrical power supply, and asecond electrically conducting pathway connecting the electrical powersupply to the anode plate. An aqueous electrolyte is provided to thecell and is disposed between the cathode and the anode. Contained in theelectrolyte is a dispersion of a complex formed from CNTs and a firstanionic polymer, and optionally a coagulant.

CNTs as used herein are generally about 0.5-2 nm in diameter where theratio of the length dimension to the narrow dimension, i.e. the aspectratio, is at least 5. In general, the aspect ratio is between 10 and2000. CNTs are comprised primarily of carbon atoms, however may be dopedwith other elements, e.g. metals. The carbon-based nanotubes of theinvention can be either multi-walled nanotubes (MWNTs) or single-wallednanotubes (SWNTs). A MWNT, for example, includes several concentricnanotubes each having a different diameter. Thus, the smallest diametertube is encapsulated by a larger diameter tube, which in turn, isencapsulated by another larger diameter nanotube. A SWNT, on the otherhand, includes only one nanotube.

CNTs may be produced by a variety of methods, and are additionallycommercially available. Methods of CNT synthesis include laservaporization of graphite [A. Thess et al, Science 273, 483 (1996)], arcdischarge [C. Journet et al, Nature 388, 756 (1997)] and HiPCo (highpressure carbon monoxide) process [P. Nikolaev et al, Chem. Phys. Lett.313, 91-97 (1999)]. Chemical vapor deposition (CVD) can also be used inproducing carbon nanotubes [J. Kong et al, Chem. Phys. Lett. 292,567-574 (1998); J. Kong et al, Nature 395, 878-879 (1998); A. Cassell etal, J. Phys. Chem. 103, 6484-6492 (1999); H. Dai et al, J. Phys. Chem.103, 11246-11255 (1999)]. Additionally CNTs may be grown via catalyticprocesses both in solution and on solid substrates [Yan Li et al, Chem.Mater.; 2001; 13(3); 1008-1014); (N. Franklin and H. Dai, Adv. Mater.12, 890 (2000); A. Cassell et al, J. Am. Chem. Soc. 121, 7975-7976(1999)].

A major obstacle to the use of CNTs has been the diversity of tubediameters, chiral angles, and aggregation states in nanotube samplesobtained from the various preparation methods. Aggregation isparticularly problematic because the highly polarizable, smooth-sidedfullerene tubes readily form parallel bundles or ropes with a large vander Waals binding energy. This bundling perturbs the electronicstructure of the tubes, and it confounds almost all attempts to separatethe tubes by size or type or to use them as individual macromolecularspecies.

There is provided by this invention a method for dispersing a populationof bundled carbon nanotubes by contacting the bundled nanotubes with anaqueous solution of an anionic polymer. A complex containing the anionicpolymer and the CNTs is thereby formed, but the association between theanionic polymer and the CNTs in the complex is a loose association, isformed essentially by van der Waals bonds or some other non-covalentmeans, and is not formed through the interaction of specificfunctionalized groups. The structural integrity of the CNTs is thereforeretained, but the complexes they form with the anionic polymers becomesuspended in a dispersion in the electrolyte.

A variety of anionic polymers may thus be used as dispersants for thepurpose of dispersing CNTs in an aqueous solution by facilitating theformation of the polymer/CNT complex, but a preferred polymer for usefor such purpose is a nucleic acid, particularly a stabilized solutionof nucleic acid molecules. Nucleic acids are very effective indispersing CNTs by the formation of nanotube-nucleic acid complexesbased on non-covalent interactions between the nanotube and the nucleicacid molecule. The method of this invention therefore includes a methodfor the dispersion of bundled CNTs by contacting the nanotubes with asolution of anionic polymers such as nucleic acid molecules.

In the following discussion of the use of nucleic acid molecules to formcomplexes with and thereby disperse CNTs, the following terms andabbreviations are used:

“cDNA” means complementary DNA

“PNA” means peptide nucleic acid

“SEM” means scanning electron microscopy

“ssDNA” means single stranded DNA

“tRNA” means transfer RNA

“CNT” means carbon nanotube

“MWNT” means multi-walled nanotube

“SWNT” means single walled nanotube

“TEM” means transmission electron microscopy

A “nucleic acid molecule” is defined as a polymer of RNA, DNA, orpeptide nucleic acid (PNA) that is single- or double-stranded,optionally containing synthetic, non-natural or altered nucleotidebases. A nucleic acid molecule in the form of a polymer of DNA may becomprised of one or more segments of cDNA, genomic DNA or synthetic DNA.

The letters “A”, “G”, “T”, “C” when referred to in the context ofnucleic acids will mean the purine bases adenine (C₅H₅N₅) and guanine(C₅H₅N₅O) and the pyrimidine bases thymine (C₅H₆N₂O₂) and cytosine(C₄H₅N₃O), respectively.

The term “peptide nucleic acids” refers to a material having stretchesof nucleic acid polymers linked together by peptide linkers.

A “stabilized solution of nucleic acid molecules” refers to a solutionof nucleic acid molecules that are solubilized and in a relaxedsecondary conformation.

A “nanotube-nucleic acid complex” means a composition comprising acarbon nanotube loosely associated with at least one nucleic acidmolecule. Typically the association between the nucleic acid and thenanotube is by van der Waals bonds or some other non-covalent means.

The term “agitation means” refers to a devices that facilitate thedispersion of nanotubes and nucleic acids. A typical agitation means issonication.

The term “denaturant” refers to substances effective in the denaturationof DNA and other nucleic acid molecules.

Standard recombinant DNA and molecular biology techniques used here arewell known in the art and are described by Sambrook, J., Fritsch, E. F.and Maniatis, T., Molecular Cloning: A Laboratory Manual, SecondEdition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.(1989) (hereinafter “Maniatis”); by Silhavy, T. J., Bennan, M. L. andEnquist, L. W., Experiments with Gene Fusions, Cold Spring HarborLaboratory Cold Press Spring Harbor, N.Y. (1984); and by Ausubel, F. M.et al., Current Protocols in Molecular Biology, published by GreenePublishing Assoc. and Wiley-Interscience (1987).

Nucleic acid molecules as used in a method of this invention may be ofany type and from any suitable source and include but are not limited toDNA, RNA and peptide nucleic acids. The nucleic acid molecules usedherein may be generated by synthetic means or may be isolated fromnature by protocols well known in the art (Sambrook supra). The nucleicacid molecules may be either single stranded or double stranded and mayoptionally be functionalized at any point with a variety of reactivegroups, ligands or agents. Functionalization of nucleic acids is not,however, required for their association with CNTs for the purpose ofdispersion, and most of the nucleic acids used herein for dispersion dolack functional groups and are therefore referred to herein as“unfunctionalized”.

Peptide nucleic acids (PNA) are particularly useful herein fordispersion as they possess the double functionality of both nucleicacids and peptides. Methods for the synthesis and use of PNA's are wellknown in the art, see for example Antsypovitch, S. I., Peptide nucleicacids: Structure, Russian Chemical Reviews (2002), 71(1), 71-83.

The nucleic acid molecules used herein may have any composition of basesand may even consist of stretches of the same base (poly A or polyT forexample) without impairing the ability of the nucleic acid molecule todisperse the bundled CNTs. Preferably the nucleic acid molecules will beless than about 2000 bases where less than 1000 bases is preferred andwhere from about 5 bases to about 1000 bases is most preferred.Generally the ability of nucleic acids to disperse CNTs appears to beindependent of sequence or base composition, however there is someevidence to suggest that the less G-C and T-A base-pairing interactionsin a sequence, the higher the dispersion efficiency, and that RNA andvarieties thereof is particularly effective in dispersion and is thuspreferred herein. Nucleic acid molecules suitable for use herein includewithout limitation those having the general formula:

-   -   1. An wherein n=1-2000;    -   2. Tn wherein n=1-2000;    -   3. Cn wherein n=1-2000;    -   4. Gn wherein n=1-2000;    -   5. Rn wherein n=1-2000, and wherein R may be either A or G;    -   6. Yn wherein n=1-2000, and wherein Y may be either C or T;    -   7. Mn wherein n=1-2000, and wherein M may be either A or C;    -   8. Kn wherein n=1-2000, and wherein K may be either G or T;    -   9. Sn wherein n=1-2000, and wherein S may be either C or G;    -   10. Wn wherein n=1-2000, and wherein W may be either A or T;    -   11. Hn wherein n=1-2000, and wherein H may be either A or C or        T;    -   12. Bn wherein n=1-2000, and wherein B may be either C or G or        T;    -   13. Vn wherein n=1-2000, and wherein V may be either A or C or        G;    -   14. Dn wherein n=1-2000, and wherein D may be either A or G or        T; and    -   15. Nn wherein n=1-2000, and wherein N may be either A or C or T        or G.

In addition to the combinations listed above, any of these sequences mayhave one or more deoxyribonucleotides replaced by ribonucleotides (i.e.RNA or RNA/DNA hybrid) or one or more sugar-phosphate linkages replacedby peptide bonds (i.e. PNA or PNA/RNA/DNA hybrid).

Nucleic acid molecules as used herein may be stabilized in a suitablesolution. It is preferred that the nucleic acid molecules be in arelaxed secondary conformation and only loosely associated with eachother to allow for the greatest contact by individual strands with theCNTs. Stabilized solutions of nucleic acids are common and well known inthe art (see Sambrook, supra) and typically include salts and bufferssuch as sodium and potassium salts, and TRIS (Tris(2-aminoethyl)amine),HEPES (N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid), andMES(2-(N-Morpholino)ethanesulfonic acid. Preferred solvents forstabilized nucleic acid solutions are those that are water misciblewhere water is most preferred. The process of dispersion may be improvedwith the optional addition of nucleic acid denaturing substances to thesolution. Common denaturants include but are not limited to formamide,urea and guanidine. A non-limiting list of suitable denaturants may befound in Sambrook, supra.

To prepare a dispersion according to a method hereof, one or morenucleic acid molecules may be contacted with a population of bundledcarbon nanotubes. It is preferred, although not required, contact bemade in the presence of an agitation means of some sort. Typically theagitation means employs sonication, but may also include devices thatproduce high shear mixing of the nucleic acids and CNTs (i.e.homogenization), or any combination thereof. Upon agitation, the CNTswill become dispersed and will form nanotube-nucleic acid complexescomprising at least one nucleic acid molecule loosely associated withthe CNT by hydrogen bonding or some other non-covalent means.

Temperature during the process of contacting CNTs with a nucleic acidmay have an effect on the efficacy of the dispersion. Mixing at roomtemperature or higher has been seen to give longer dispersion timeswhereas mixing at temperatures below room temperature (23° C.) has beenseen to give more rapid dispersion times, and temperatures of about 4°C. are preferred. The dispersion of CNTs by contact with nucleic acidmolecules is also described in US 2004/0132072 and US 2004/0146904, eachof which is by this reference incorporated in its entirety as a parthereof for all purposes.

In addition to the nucleic acid molecules described above, one or moreother anionic polymers may be used for the purpose of preparing anaqueous dispersion of CNTs. Examples of other anionic polymers that havebeen found useful in the preparation of a dispersion of CNTs includewithout limitation ionized poly(acrylic acid) (“PAA”) or ionizedethylene/(meth)acrylic acid copolymer (“EAA” or “EMAA”), either of whichmay be neutralized with cations such as Na⁺, K⁺, NH₄ ⁺ or Cr⁺; styrenicionomers such as styrene/sodium styrene sulfonate copolymer (PSS) orstyrene/sodium styrene methacrylate copolymer; and ionizedtetrafluoroethylene/sulfonic acid copyolymers such as Nafion™ copolymer(from DuPont) in which the sulfonic acid group in atetrafluoroethylene/perfluorovinyl ether copolymer may be sodiumneutralized. As indicated above with respect to nucleic acid molecules,ultrasonication or other mixing means may be applied to facilitate thedispersion of CNTs in an aqueous solution of one or more of the anionicpolymers discussed in this paragraph.

In one embodiment, deposition on the anode plate of the cell of thecomplexes formed from CNTs and molecules of an anionic polymer, asdispersed in the electrolyte solution contained in the cell, will befacilitated by the presence therein of the optional coagulant. Thecoagulant will neutralize the negative charge on the anionic polymer inthe complex. As the population of anionic polymer/CNT complexes has beenmaintained in dispersion primarily by the repulsion of one negativelycharged complex from another (or by the repulsion of positively chargeddouble layers surrounding the complexes), neutralization of thosenegative charges (or compression of the double layer) by the coagulantwill remove the force enabling the population of complexes to remain indispersion in the electrolyte solution. As the action of the coagulantto neutralize the complexes occurs in close proximity to the anodeplate, the complexes (as no longer dispersed) will in varying degreesundergo a transition from solution phase to solid phase, becomeprogressively aggregated and agglomerated (similar to the formation offloccules and flocs), and then be collected and deposited on the surfaceof the anode plate. In addition to the CNT complexes, the material asdeposited on the plate may include coagulant residue.

If first and second anionic polymers are present in the electrolytesolution, such as a first polymer that forms a complex with CNTs and asecond polymer that does not or is more loosely associated with CNTsthan the first polymer, they may become deposited on the surface of theanode at the same time. The first polymer may, for example, be depositedin a matrix of the second polymer. If additional materials needed toenhance the usefulness and performance of the anode plate in a fieldemission device, such as conductive or functionalized particles, arepresent in the electrolyte solution, they may be deposited on the anodeplate at the same time as the anionic polymer/CNT complexes. FIG. 2shows a typical example of the type of film formed by such deposition onthe anode plate, which film has good uniformity of evenly deposited,well-adhered material all across its surface.

Coagulants suitable for use herein for the purpose of neutralizing ananionic polymer/CNT complex include inorganic coagulants such astrivalent cations formed from metals including Group VIII/VIIIA metalssuch as iron, cobalt, ruthenium or osmium. As a trivalent cation can beup to as much as ten times more effective in neutralizing the complexthan a divalent cation, a convenient way to provide the coagulant is tosupply a divalent cation such as tris(2,2′-bipyridyl)dichloro-ruthenium(II) to the electrolyte solution wherein the 2⁺ cation is oxidized to a3⁺ valence by the loss of electrons to the anode plate. A schematicrepresentative of this mechanism is shown in FIG. 1.

In an alternative embodiment, however, a coagulant is not used where theanode plate is formed from a metal such as silver or nickel. In suchcase, the metal on the plate dissolves in the electrolyte solution, andthe charge on an anionic polymer/CNT complex is neutralized by cationsformed from metal atoms that have gone into solution from the solidmetal from which the plate if formed.

The method hereof is generally performed by operation of the cell atlower potential such as less than about 5 volts, or from about 2 to lessthan about 5 volts, or from about 2 volts to about 3 volts. Thickness ofthe deposited film is to a large extent directly related to length ofdeposition time. A deposition time in the range of about 1 to about 10minutes, or in the range of about 1 to about 2 minutes, may be used. Apositive potential is maintained at the anode plate relative to thecathode of the cell.

The method hereof may be used to produce a film in which the depositedmaterial is deposited in a pre-determined pattern. This may beaccomplished by patterning the surface of the plate used as the anodeusing conventional photoimaging techniques. Thus a photoresist may beactivated through a mask and then developed to provide on the surface ofthe anode a pattern such as an array of circular wells. As the anionicpolymer/CNT complexes are aggregated and settle out of solution, theyare deposited only in the holes, and the photoresist may be removed.This provides a patterned CNT film, with the anode plate serving as asubstrate for the film, for use by installation in a field emissiondevice.

After completion of the deposition of CNT complex material on the anodeplate in the cell, the plate may be removed from the cell, rinsed, driedand installed in such condition in a field emission device for use aspart of the cathode assembly therein to provide electron emission.Alternatively, however, the plate may be baked and/or fired beforeinstallation in a field emission device to melt the deposited polymer(s)and utilize them in that form as an adhesive to more securely anchor theCNTs to the surface of the plate, resulting in a CNT-containing filmwith excellent abrasion resistance.

In the field emission device into which the above described plate coatedwith deposited material may be installed, an electron emitting materialis disposed on a cathode and, when energized, bombards an anode withelectrons. The electron emitting material may be an acicular substancesuch as carbon, a semiconductor, a metal or mixtures thereof. As usedherein, “acicular” means particles with aspect ratios of 10 or more.Typically, glass frit, metallic powder or metallic paint or a mixturethereof is used to attach the electron emitting material to a substratein the cathode assembly.

Acicular carbon as used as the electron emitting material may be ofvarious types, but carbon nanotubes are the preferred acicular carbonand single wall CNTs are especially preferred. Carbon fibers grown fromthe catalytic decomposition of carbon-containing gases over small metalparticles are also useful as acicular carbon, and other examples ofacicular carbon are polyacrylonitrile-based (PAN-based) carbon fibersand pitch-based carbon fibers.

Various processes can be used to attach an electron emitting material toa substrate. The means of attachment must withstand and maintain itsintegrity under the conditions of manufacturing the apparatus into whichthe field emitting cathode is placed and under the conditionssurrounding its use, e.g. typically vacuum conditions and temperaturesup to about 450° C. A preferred method is to screen print a pastecomprised of the electron emitting material and glass frit, metallicpowder or metallic paint or a mixture thereof onto a substrate in thedesired pattern and to then fire the dried patterned paste. For a widervariety of applications, e.g. those requiring finer resolution, thepreferred process comprises screen printing a paste which furthercomprises a photoinitiator and a photohardenable monomer,photopatterning the dried paste and firing the patterned paste.

The substrate can be any material to which the paste composition willadhere. If the paste is non-conducting and a non-conducting substrate isused, a film of an electrical conductor to serve as the cathodeelectrode and provide means to apply a voltage to the electron emittingmaterial will be needed. Silicon, a glass, a metal or a refractorymaterial such as alumina can serve as the substrate. For displayapplications, the preferable substrate is glass and soda lime glass isespecially preferred. For optimum conductivity on glass, silver pastecan be pre-fired onto the glass at 500-550° C. in air or nitrogen, butpreferably in air. The conducting layer so-formed can then beover-printed with the emitter paste.

The paste used for screen printing typically contains the electronemitting material, an organic medium, solvent, surfactant and either lowsoftening point glass frit, metallic powder or metallic paint or amixture thereof. The role of the medium and solvent is to suspend anddisperse the particulate constituents, i.e. the solids, in the pastewith a proper rheology for typical patterning processes such as screenprinting. There are many organic media known for use for such purposeincluding cellulosic resins such as ethyl cellulose and alkyd resins ofvarious molecular weights. Butyl carbitol, butyl carbitol acetate,dibutyl carbitol, dibutyl phthalate and terpineol are examples of usefulsolvents. These and other solvents are formulated to obtain the desiredviscosity and volatility requirements.

A glass frit that softens sufficiently at the firing temperature toadhere to the substrate and to the electron emitting material is alsoused. A lead or bismuth glass frit can be used as well as other glasseswith low softening points such as calcium or zinc borosilicates. If ascreen printable composition with higher electrical conductivity isdesired, the paste may also contain a metal, for example, silver orgold. The paste typically contains about 40 wt % to about 80 wt % solidsbased on the total weight of the paste. These solids include theelectron emitting material and glass frit and/or metallic components.Variations in the composition can be used to adjust the viscosity andthe final thickness of the printed material.

The emitter paste is typically prepared by three-roll milling a mixtureof the electron emitting material, organic medium, surfactant, solventand either low softening point glass frit, metallic powder or metallicpaint or a mixture thereof. The paste mixture can be screen printedusing, for example, a 165-400-mesh stainless steel screen. The paste canbe deposited as a continuous film or in the form of a desired pattern.When the substrate is glass, the paste is then fired at a temperature ofabout 350° C. to about 550° C., preferably at about 450° C. to about525° C., for about 10 minutes in nitrogen. Higher firing temperaturescan be used with substrates which can endure them provided theatmosphere is free of oxygen. However, the organic constituents in thepaste are effectively volatilized at 350-450° C., leaving a layer of thecomposite of the electron emitting material and glass and/or metallicconductor. The electron emitting material appears to undergo noappreciable oxidation or other chemical or physical change during thefiring in nitrogen.

If the screen-printed paste is to be photopatterned, the paste may alsocontain a photoinitiator, a developable binder and a photohardenablemonomer comprised, for example, of at least one addition polymerizableethylenically unsaturated compound having at least one polymerizableethylenic group. Typically, a paste prepared from an electron emittingmaterial such as CNTs, silver and glass frit will contain about 0.01-6.0wt % nanotubes, about 40-75 wt % silver in the form of fine silverparticles and about 3-15 wt % glass frit based on the total weight ofthe paste.

The anode of the field emission device is an electrode coated with anelectrically conductive layer. When the field emission device is used ina display device where the cathode contains an array of pixels of thethick film paste deposits described above, the anode in the displaydevice may comprise phosphors to convert incident electrons into light.The substrate of the anode would also be selected to be transparent sothat the resulting light could be transmitted. From the cathode assemblyand anode, a sealed unit is constructed in which the cathode assemblyand anode are separated by spacers, and there is an evacuated spacebetween the anode and the cathode. This evacuated space needs to beunder partial vacuum so that the electrons emitted from the cathode maytransit to the anode with only a small number of collisions with gasmolecules. Frequently the evacuated space is evacuated to a pressure ofless than 10⁻⁵ Torr.

Such a field emission device is useful in a variety of electronicapplications, e.g. vacuum electronic devices, flat panel computer andtelevision displays, back-light source for LCD displays, emission gateamplifiers, and klystrons and in lighting devices. For example, flatpanel displays having a cathode using a field emission electron source,i.e. a field emission material or field emitter, and a phosphor capableof emitting light upon bombardment by electrons emitted by the fieldemitter have been proposed. Such displays have the potential forproviding the visual display advantages of the conventional cathode raytube and the depth, weight and power consumption advantages of the otherflat panel displays. The flat panel displays can be planar or curved.U.S. Pat. Nos. 4,857,799 and 5,015,912 disclose matrix-addressed flatpanel displays using micro-tip cathodes constructed of tungsten,molybdenum or silicon. WO 94-15352, WO 94-15350 and WO 94-28571 discloseflat panel displays wherein the cathodes have relatively flat emissionsurfaces. These devices are also described in US 2002/0074932, which isby this reference incorporated in its entirety as a part hereof for allpurposes.

Materials as used in the process hereof may be made by processes knownin the art, or are available commercially from suppliers such as AlfaAesar (Ward Hill, Mass.), City Chemical (West Haven, Conn.), FisherScientific (Fairlawn, N.J.), Sigma-Aldrich (St. Louis, Mo.) or StanfordMaterials (Aliso Viejo, Calif.).

The advantageous attributes and effects of this invention may be seen ina series of examples (Examples 1˜5), as described below. The embodimentson which the examples are based are representative only, and theselection of those embodiments to illustrate the invention does notindicate that materials, conditions, specifications, components,reactants, techniques and protocols not described in these examples arenot suitable for practicing this invention, or that subject matter notdescribed in these examples is excluded from the scope of the appendedclaims and equivalents thereof.

EXAMPLES

150 mg of laser-ablated CNTs (from CNI, Houston, Tex.) was mixed with 30mg yeast RNA (from Sigma Aldrich) in 15 mL of 1×TBE [tris borate(ethylenediaminetetraacetic acid)] buffer (from Sigma Aldrich). Themixture was sonicated with a probe sonicator at a power level of 20 Wfor 30 min. The resulting dispersion was mixed with two other componentsaccording to the following table (Table 1) to make up 100 mL ofdeposition solution. Ru²⁺(bipy)₃ as used in the deposition solution istris(2,2′-bipyridyl)dichloro-ruthenium (II) and is obtained from SigmaAldrich. EMMA is ethylene/methacrylic acid ionomer obtained from DuPontas Surlyn™ ionomer.

TABLE 1 Composition of deposition solution Component Stock conc. Volumeadded Final conc. CNT 10 mg/mL 4 mL 0.04% dispersion EMMA 10 mg/mL 2 mL0.02% Ru²⁺⁽bipy)₃ 10 mM   2 mL 0.2 mM water 92 mL 

Example 1

A 2′×2′ stainless steel plate (used as the cathode) and a 2′×2′ indiumtin oxide (“ITO”) plate (used as the anode) were inserted in a parallelfashion into a rectangular electrochemical cell (the configuration forwhich is shown in FIG. 3). The cell was charged with 15 mL of thedeposition solution as the electrolyte. A potential difference of 3.2 Vwas applied between the two electrodes. After 1 minute, the depositionwas stopped, and the ITO plate was taken out of the cell, rinsed withdeionized water and dried in air. Uniform deposition of material on theplate was obtained as shown in FIG. 2.

Example 2

A photoresist (PR) patterned indium tin oxide (ITO) substrate (2′×2′)was used as the anode. The PR layer defines an array of circular wellswith 20 um diameter. The wells expose the surface of the ITO plate forCNT deposition. Before electrodeposition, the PR coated ITO plate wasdipped into a solution of 0.01% Triton X-100, taken out and dried byblowing N₂ gas. This assists with coating the hydrophobic PR layer witha thin hydrophilic layer for better wetting. After this treatment, a2′×2′ stainless steel plate (used as the cathode) and the PR-coated ITOplate (used as the anode) were inserted in a parallel fashion into thesame type of electrochemical cell as used in Example 1. The cell wascharged with 15 mL of the deposition solution. An AC potential (100 Hzsquare wave with 0 to 3.5 V peak-to-peak voltage and 50% duty cycle) wasapplied between the two electrodes. After 1 minute, the deposition wasstopped, and the ITO plate was taken out of the cell, rinsed withdeionized water and dried in air. The PR layer was removed by treatmentwith an acetone solvent. Good uniformity of deposition of CNT materialin the exposed wells was obtained as shown in FIG. 4.

Example 3

The dried plate obtained from Example 1 and depicted in FIG. 2 was thenfired in nitrogen for 10 minutes at 420° C. A piece of adhesive tape wasthen laminated over the CNT film and subsequently removed. This process,commonly referred to as “activation”, is known to fracture the filmsurface exposing and lifting the CNT filaments off the substrate surfaceto dramatically enhance electron field emission. A diode field emissiondevice was then assembled by using the CNT film coated ITO substrate asa cathode. Opposite to this “activated” cathode, an anode plateconsisting of an ITO coated glass substrate with a phosphor coating wasmounted. Electrically insulating spacers 1 mm thick were used tomaintain a distance between the cathode and anode substrates. Electricalcontact was made to the cathode and anode electrodes using silver paintand copper tape to complete the diode device. The device was mounted ina vacuum chamber which was evacuated to a pressure of <1×10⁻⁵ Torr. Apulsed square wave with a repetition rate of 60 Hz and a pulse width of60 μs was applied to the anode electrode. The cathode electrode wasmaintained at ground potential. At an anode voltage of 2 kV, an anodecurrent of 200 μA was obtained. An image of phosphor illumination byelectrons emitted by this device is shown in FIG. 5.

Example 4

The dried plate obtained from Example 2 and depicted in FIG. 4 was firedin nitrogen for 10 minutes at 420° C. as described in Example 3. The CNTdot surfaces were activated with a piece of adhesive tape as describedin Example 3. A diode field emission device was then assembled by usingthe CNT dot covered ITO substrate as a cathode and an ITO coated glasssubstrate with a phosphor coating as anode. Glass spacers 0.22 mm thickwere used in this example to maintain a distance between the cathode andanode substrates. The device was mounted in a vacuum chamber which wasevacuated to a pressure of <1×10⁻⁵ Torr. A pulsed square wave with arepetition rate of 60 Hz and a pulse width of 60 μs was applied to theanode electrode. The cathode electrode was maintained at groundpotential. When the pulsed anode voltage reached 800 V, an average anodecurrent of 5 μA was measured. As the pulse anode voltage was increased,increasing anode current was measured. At an anode voltage of 925 V, ananode current of 40 μA was obtained. FIG. 6 shows a plot of the recordedanode current and anode voltage values from this field emission device.An image of phosphor illumination by electrons emitted by this device,operating at 975 V anode voltage and 80 μA anode current, is shown inFIG. 7. Each rectangular illuminated pixel on the anode was produced byan array of multiple CNT dots on the cathode.

Example 5

Instead of a plain ITO coated glass substrate as was made in Example 2and used in Example 4, the method hereof was used to deposit CNT dots ona top-gate triode substrate. A top-gate triode substrate typicallyconsists of two conductive layers between which is disposed aninsulating layer. In this example, an ITO coated glass substrate wasused as the substrate for the top-gate triode, using the ITO layer asthe cathode. An insulating dielectric layer was deposited on top of theITO layer. A metallic gate electrode layer was deposited on thedielectric layer. In addition, an array of circular wells was etchedthrough the metal and dielectric layers, using a photoresist (“PR”) andmask, exposing the ITO surface. As in Example 2, the array of circularwells defined a pattern on the PR layer that covered the triodeassembly. The openings of the wells in the PR had a smaller diameterthan the diameter of the wells that extended through the metal anddielectric layer, but the circumference of the smaller well wasconcentric with that of the larger. Using procedures similar to thosedescribed in Examples 2 and 4, CNT dots were deposited on the ITOsurface, fired and activated.

Opposite the activated triode cathode, an anode plate consisting of anITO coated glass substrate with a phosphor coating was mounted. Spacers3 mm thick were used to maintain the distance between the cathode andanode substrates. Electrical contact was made to the ITO cathodeelectrode, metal gate electrode, and ITO anode electrode using silverpaint and copper tape to complete a top-gate triode device. The devicewas mounted in a vacuum chamber which was evacuated to a pressure of<1×10⁻⁵ Torr. A DC voltage of 3 kV was applied to the anode electrode. Apulsed square wave with a repetition rate of 120 Hz and a pulse width of30 μs was applied to the gate electrode. The cathode electrode wasmaintained at ground potential. When the pulsed gate voltage reached 70V, an average anode current density of 5.0 μA/cm² was measured. An imageof phosphor illumination by electrons emitted by this triode device isshown in FIG. 8.

Features of certain of the devices of this invention are describedherein in the context of one or more specific embodiments that combinevarious such features together. The scope of the invention is not,however, limited by the description of only certain features within anyspecific embodiment, and the invention also includes (1) asubcombination of fewer than all of the features of any describedembodiment, which subcombination may be characterized by the absence ofthe features omitted to form the subcombination; (2) each of thefeatures, individually, included within the combination of any describedembodiment; and (3) other combinations of features formed by groupingonly selected features of two or more described embodiments, optionallytogether with other features as disclosed elsewhere herein.

In this specification, unless explicitly stated otherwise or indicatedto the contrary by the context of usage, where an embodiment of thesubject matter hereof is stated or described as comprising, including,containing, having, being composed of or being constituted by or ofcertain features or elements, one or more features or elements inaddition to those explicitly stated or described may be present in theembodiment. An alternative embodiment of the subject matter hereof,however, may be stated or described as consisting essentially of certainfeatures or elements, in which embodiment features or elements thatwould materially alter the principle of operation or the distinguishingcharacteristics of the embodiment are not present therein. A furtheralternative embodiment of the subject matter hereof may be stated ordescribed as consisting of certain features or elements, in whichembodiment, or in insubstantial variations thereof, only the features orelements specifically stated or described are present.

1. A method for the deposition of carbon nanotubes, comprising: (a)providing an electrochemical cell that comprises a cathode, an anodeplate, a first electrically conducting pathway connecting the cathode toan electrical power supply, and a second electrically conducting pathwayconnecting the electrical power supply to the anode plate; (b) providingas an aqueous electrolyte disposed between the cathode and the anode adispersion of a complex formed from carbon nanotubes and a first anionicpolymer; and (c) applying a voltage to the electrochemical cell todeposit the complex on the anode.
 2. A method according to claim 1wherein the aqueous electrolyte further comprises a coagulant.
 3. Amethod according to claim 2 wherein coagulant residue is deposited onthe anode together with the complex.
 4. A method according to claim 1wherein the first polymer comprises a nucleic acid molecule.
 5. A methodaccording to claim 1 wherein the first polymer comprises RNA.
 6. Amethod according to claim 1 wherein the electrolyte further comprises asecond anionic polymer.
 7. A method according to claim 6 wherein thesecond ionic polymer comprises a styrenic ionomer or an ionizedethylene/(meth)acrylic acid copolymer.
 8. A method according to claim 6wherein the complex, as deposited on the anode, is deposited in a matrixof the second anionic polymer.
 9. A method according to claim 7 whereinthe first polymer comprises a nucleic acid molecule.
 10. A methodaccording to claim 7 wherein the first polymer comprises RNA.
 11. Amethod according to claim 1 further comprising a step of removing theanode plate from the cell, and installing it in a field emission device.12. A film comprising a substrate and, disposed on the substrate, (a)coagulant residue, and (b) a complex formed from carbon nanotubes and afirst anionic polymer.
 13. A method according to claim 12 wherein thefirst polymer comprises a nucleic acid molecule.
 14. A method accordingto claim 12 wherein the first polymer comprises RNA.
 15. A filmaccording to claim 12 wherein there is further disposed on the substratea second anionic polymer.
 16. A method according to claim 15 wherein thesecond ionic polymer comprises a styrenic ionomer or an ionizedethylene/(meth)acrylic acid copolymer.
 17. A method according to claim15 wherein the first polymer comprises a nucleic acid molecule.
 18. Amethod according to claim 15 wherein the first polymer comprises RNA.19. A cathode assembly for a field emission device comprising a filmaccording to claim
 1. 20. A field emission device comprising a cathodeassembly according to claim 19.